In recent years, facilities for pursuing the convenience or comfort of a user in a vehicle have dramatically increased. However, some of the facilities become noise sources for a musical signal of an audio system. For example, an inverter for connecting an AC device, such as a personal computer, to a vehicle or an inverter for a hybrid engine generates a large amount of noise. In addition, for example, a gadget, such as a wireless battery charger of a smart phone, generates noise in an AM band or an FM band. Therefore, it is necessary to perform noise removal for various types of noise in an audio system which is used in a vehicle.
In a noise removal process disclosed in US 2006-0030287A, as a technology for performing noise removal, noise removal is performed by performing an adaptive filtering process using a modulation signal which is received from an antenna (that is, a signal acquired by performing FM or AM modulation on an audio signal in order to enable wireless transmission) and a noise reference signal which is received from another antenna.
In addition, in a noise removal process disclosed in EP 2237270A, a noise reference signal is acquired by performing an adaptive filtering process on each of two audio signals, and noise removal is performed by performing the adaptive filtering process again using one of the two audio signals and the noise reference signal.
However, in the noise removal processes disclosed in US 2006-0030287A and EP 2237270A, the type of noise is not considered and it may be implicitly considered that Gaussian noise is supposed to be present. Therefore, when impulsive noise is included in a signal, there is a possibility that noise is not sufficiently removed or the calculation of noise removal does not converge. Therefore, it is difficult to mention that the technologies disclosed in US 2006-0030287A and EP 2237270A cope with removal of various types of noise.